-3+(-6)+(-12) .... S12=

Deret Geometri

r = -6/-3 = 2

Sn
= a(1- r^n) / 1-r

S12
= -3(1-2^12)/ 1-2
= -3 (1 - 4096)-1
= 3(-4095)
= - 12,285